Electric motors having a horsepower rating greater than one horsepower are required under the National Electric Code to have some means of disconnecting the power source to the motor in the event the motor windings exceed a predetermined temperature. These motor protection devices take many forms and usually require electrical circuitry external to the motor.
Commercial dishwashers frequently employ motors having horsepower rating requiring motor protection devices. These dishwashers also include means for heating a reservoir of water, means for sensing the temperature of the water and an electrical control circuit for controlling the water heating means.
Domestic dishwashers also employ temperature responsive elements for protecting the pump motor from excessive temperatures and for controlling the temperature of the heater used in the washing and rinsing cycles of the machine. Heretofore, thermal responsive devices, such as bimetallic elements, have been employed for all of the temperature control functions in these domestic dishwashers.
In both domestic and commercial dishwashers, the motor protection circuit and the water temperature control circuit have often been designed separately and on an individual basis for each model of dishwasher manufactured. This manufacturing technique is expensive due to the large number of replacement parts necessary and the time spent in training repairmen to perform field service on the entire line of equipment produced by one manufacturer.
For example, one type of motor protection device is a thermally responsive element which responds to the temperature of a heater inserted in series with and responding to the current in the motor circuit. Theoretically, the temperature of the motor is related to the current input to the motor windings, and therefore the temperature of the heater is sensed by the thermally responsive device which will open the circuit to the motor in the event of overload. Obviously, this technique can only approximate the temperature of the motor but cannot sense the actual temperature within the motor windings where damage from motor overtemperature is most likely to occur.
Furthermore, the heater elements are selected on the basis of the current drawn by the motor, thus requiring different heaters for motors operated from different power sources and motors of varying horsepower ratings. This requires service personnel to stock a large number of different heater elements, and should a particular heater element be depleted from the stock at the time a serviceman is called upon to service an installed dishwasher, it is possible that he will replace the heater element with either a more or less sensitive element. If the heater element is less sensitive, it might result in overheating and possible failure of the motor at some later time; if it is too sensitive, then current to the motor might be interrupted although the motor temperature has not exceeded the specified value.